1. Field of the Invention
The present invention relates to a vehicular automatic transmission, and especially to a hydraulic control system for permitting manual shifting of the transmission.
2. Description of the Prior Art
In a conventional vehicular automatic transmission, hydraulic pressure is supplied from a hydraulic pressure source of a hydraulic control system and is then regulated. A shift lever is operated to move a manual valve so that the thus-regulated hydraulic pressure is relayed to one of several shift valves. The thus selected shift valve is operated under the control of an associated solenoid valve in accordance with vehicular running conditions such as vehicle speed and throttle position, whereby the hydraulic pressure is supplied to an associated hydraulic servomotor. The hydraulic servomotor then controls frictional engagement elements of the corresponding clutch or brake of the transmission to engage or release, thereby permitting automatic selection of a gear ratio.
In the transmission constructed as described above, the engagement or non-engagement of a second coast brake and a first and reverse brake varies depending on whether the manual valve is moved to a drive ("D") position or to a second ("S") or low ("L") position even when an automatically pre-established first or second speed is chosen. At the "L" position, engine braking is effected when running at either the first or second speeds. At the "S" position, however, no engine braking is effected in first speed but engine braking is effected in the second and third speeds. At the "D" position, on the other hand, no engine braking is applied in the first or second speeds but is applied when running in the third and fourth speeds.
With a view toward meeting the recent trend toward better responsiveness and feeling of control, positively reflecting the driver's desire or intention with respect to running performance and hence to enhance the fun of driving, research has been directed to an automatic transmission which provides the driver with the feeling of driver control associated with a manual gear shift vehicle. One example of such a proposed automatic transmission would have a gear shift "I" pattern and an "H" pattern combined side-by-side. Another example has a manual gear shift pattern provided in parallel with an "I" pattern so as to permit shift-up or shift-down by tilting the shift lever forward or rearward as in the "TIPTRONIC" of Porsche AG. Another example of such an automatic transmission has been previously proposed by the present assignee in which a manual valve is mechanically changed over following movement of a shift lever along a combined "I" and "H" pattern.
The former two examples are, however, designed to perform manual shifting only by utilizing an electrical signal in the "D" range of a conventional automatic transmission so that no engine braking can be effected in first speed in a manual mode. In the latter example, on the other hand, a manual valve in a hydraulic control system is operated to switch the range pressure among the "D", "S" and "L" range pressures even upon manual shifting, thereby bringing about the advantages that engine braking can be effectively applied in the first and second speeds and, at the same time, application of excessive engine braking during high-speed running can be avoided. However, the latter example also presents the problem that, because the manual valve is changed over along the "H" pattern, a complex mechanism for converting motion of the shift lever along the "H" pattern to the linear motion of the manual valve is required, resulting in the need for greater effort to operate the shift lever. Accordingly, the operational feeling, i.e. the feeling of precision of shift lever movement, is not fully satisfactory.